There are various proposals of a micro object detection apparatus that irradiates with light a space in which suspended micro particulate matter (hereinafter referred to as “particle(s)”) such as pollen or dust exists, detects scattered light generated at that time, and detects an amount of particles, the size of particles, kinds of particles, or the like.
For example, patent reference 1 discloses a particle sensor that includes a light source, a light reception element, and converging mirrors, to irradiate a particle with light emitted from the light source, reflect scattered light by the converging mirrors, and detect the intensity of the scattered light by the light reception element.
In the particle sensor described in patent reference 1, the two opposite converging mirrors are an elliptical mirror and a spherical mirror. A passage region where a particle that radiates scattered light passes through is located at the position of one focal point (first focal point) of the elliptical mirror. In addition, the light reception element for receiving the scattered light is located at the position of the other focal point (second focal point) of the elliptical mirror. The focal point (the center point of the curvature radius of a spherical mirror surface) of the spherical mirror is positioned at the position of the first focal point of the elliptical mirror. Accordingly, it is possible to further reflect the reflected light from the spherical mirror by the elliptical mirror and to direct the reflected light to the light reception element.